ndisc6: remove man page related code

The target-finalize target takes care of removing
$(TARGET_DIR)/usr/share/man, so there's no point in having package
specific code doing this.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>